1. Field of the Invention
The present invention relates to a process for producing hydraulic blended cement with a significantly reduced Portland clinker minerals content and with a corresponding increased amount of supplementary materials.
2. Description of the Related Art
It is well known that each ton of production of Standard Portland Cement is accompanied by the release of about one ton of carbon dioxide, and that about half of that comes from the decarbonization of limestone in the kiln and the other half from energy consumption, primarily in the kiln.
It follows that the only way in which the cement industry can achieve meaningful reductions in carbon dioxide emissions is via the reduction of Portland clinker production and increased use of fillers. The cement industry does not believe that more than about 2-3% further reduction in energy consumption is possible in Standard Portland Cement production.
The traditional methods of production of blended cement include intergrinding Portland cement clinker with different types of microfillers, e.g., blast furnace slag, fly ash, limestone, etc., mainly in rotating ball mills. Such methods do not provide more than 20-25% of Portland clinker replacement by fly ash and approximately 30-50% by blast furnace slag, without significant negative influence on the cement performance, such as a decrease of setting time, very low strength development during the curing period 0-28 days, etc. It takes up to 3 times longer time (2-3 months) to achieve a 28-day strength of traditional Portland Cement concretes. At the same time, the high volume fly ash (HVFA) cements do have significant benefits in comparison with traditional Portland cements. Concretes produced with such cements are characterized by high durability, such as a low chloride permeability, a high sulphate and alkali-silica resistance, etc., see, e.g., Malhotra, Concrete International J., Vol. 21, No. 5, May 1999, pp. 61-66. According to Malhotra, strength development of such concretes could be improved by significantly increasing the content of a binder, such as cement+a microfiller, and significantly decreasing the amount of mixed water. But such an approach requires increased dosages of water-reduced admixtures to keep an acceptable consistency of the concrete mixtures, which sharply increases the cost of the concrete.
Another method related to the present invention is a method described by the US Patent Appln. Publ. No. US 2002/0000179, for a “Method for producing a blended cementitious composition”. Said application suggests the introduction into the concrete mixtures of Portland Cement and traditional cementitious materials, such as fly ash, blast furnace slag, etc., and very fine and highly reactive rice hull ash. This gives an improvement of early age strength and chloride permeability of the concretes.
Such a method of producing concrete requires increased amounts, up to 400 kg per cubic meter, of a binder material, such as cement+fly ash+rice hull ash, in order to achieve a concrete which is comparable to Standard Portland Cement concretes at relatively low water-to-binder ratios, such as less than 0.40. These requirements result in significant technical and economical limitations for the application of that method by the ready mix concrete industry, which produces about 95% of the concrete with a cement/binder content of 250-300 kg per cubic meter and a water to cement ratio of 0.60-0.70. Introduction of that method for the mentioned concrete mix design leads to a drastic decrease of the concrete strength development and an increase of the setting time, which make it non-competitive with Standard Portland Cement concretes.